The present invention relates to structural engineering adhesive for bonding metal and fibrous-reinforced composite (e.g. sheet molding compounds (SMC), fiberglass reinforced polyesters (FRP), resin transfer molding (RTM), and the like) parts to a variety of similar and dissimilar substrates which find use in the manufacture of cars, trucks, boats, and a host of other products.
Sheet molding compound (SMC), for example, is defined (ASTM) as a molding compound in integral sheet form comprising a thermosetting resin, fibrous reinforcement, and additives required for processing or product performance, e.g., resin, catalyst, thickener, mold release agent, particulate filler, pigment, shrink control agent, etc. Fibrous reinforced polyester (FRP) comprises polyester thermosetting resins retaining fibrous reinforcement and conventional additives. These materials and others generally are known as fibrous-reinforced composites, reinforced composites, or simply composites.
One typical class of structural adhesives useful in adhering composite parts to the same and to diffeent substrates are two-part polyurethane adhesives. These adhesives are made by combining a prepolymer and a curative just before use. The ratio in which these materials are combined will vary depending upon the functionality of the prepolymer and the curative. Accurate combination of the materials requires a certain skill level of the worker and, unfortunately, there is substantial waste of adhesive during the mixing process even using automatic pumping equipment.
Another typical class of structural adhesives useful in adhering metal parts to the same and to different substrates (e.g. composites) are epoxy adhesives. Epoxy adhesive compositions typically contain a polyfunctional epoxy resin and are cured by addition of a curative which typically is provided in a separate package. The rate of cure and product characteristics are influenced by the choice of curing agent, which itself is influenced by the make-up of the adhesive composition, as dictated by the final properties desired by the user.
Structural adhesives are used by application to the surface of a part made of, e.g. metal, and positioning a surface of second part (of the same or different material) over the adhesively-covered metal surface. Since the parts often have uneven surfaces, it is desirable that the adhesive possess the ability to fill the resulting voids of varying depth. It is important that the adhesive remain uncured and fluid for sufficient time to permit placing of the second substrate into contact with the adhesive. An adhesive which hardens too quickly does not permit flexilibity in the assembly line process. Thus, the length of time the adhesive is fluid is measured and is referred to as "open time". The adhesive may be cured by placing the adhered parts in an oven maintained at, e.g., 70.degree.-190.degree. C. for e.g., 5 minutes or less to cure or harden the adhesive, but preferably the adhesive may be cured by letting it stand at room temperature for one to several days, e.g. 3 days.
Representative epoxy structural adhesive compositions can be found in, for example, U.S. Pat. Nos. 4,707,518, 4,803,232, 4,623,702, 4,316,979, 3,101,326, 4,689,389, 3,882,091, and 4,017,554, the disclosures of which are expressly incorporated herein by reference. Various combinations of epoxy resins, rubber modifiers, amine curing agents, amide curing agents, Lewis acids, mercaptan curing agents, etc. have been proposed for formulating high strength adhesive compositions. A major deficiency in these adeshives is that none has proven to contain an acceptable balance of (1) room temperature curing and rapid gel times, (2) high lap shear strength, (3) high T-peel strength (4) adhesion to oil contaminated surfaces, (5) attainment of full cure within a relatively short time period, and (6) resistance to water. It is to an adhesive composition that contains such acceptable balance of properties that the present invention is addressed.